This invention is directed to reflective filters and in particular to reflective filters made of optical fibers having light induced refractive index perturbations.
In present optical fiber communication systems only a small fraction of the total available information carrying capacity is utilized. More effective use of the available bandwidth is obtained by implementing optical communication systems employing wavelength division multiplexing (WDM). The implementation of WDM systems requires components for wavelength multiplexing and demultiplexing such as optical combiners, wavelength selective filters and reflectors.
Two general approaches are available for providing wavelength selective devices for fiber optic WDM systems-microoptics and thin-film integrated optics. In the microoptic approach, miniaturized versions of the standard optical components (prisms, gratings, lens, beamsplitters, interference filters, etc.) are fabricated and used to carry out WDM. Whereas in the thin-film integrated optic approach the optical components are fabricated in planar thin films using suitable deposition techniques.
The microoptic approach is a straightforward extension of present optical component technology to small sizes to be compatible with the optical fiber size. Since the fabrication techniques are usually labour intensive and not easily adapted to mass production, the component cost is high. In addition the wavelength selective components such as prisms and gratings have low spectral resolution thus severely restricting the number of different wavelengths that can be multiplexed or demultiplexed on a single fiber.
In the thin-film integrated optic approach the fabrication process for the components is amenable to large scale production resulting in a low component costs. The principal limitation of this approach is the high optical loss occurring in planar thin-films which limits the effective length of the devices to less than 1 cm and thus places a restriction on the optical resolution of the devices. A further difficulty is the optical mode mismatch that results when coupling light from a waveguide with circular geometry into one with rectangular geometry and vice versa.